1. Field of the Invention
The present invention relates to a technique which can be preferably used for a thin inner rotor motor for rotatably driving a medium which is used in a floppy disk driving device or the like, for example.
2. Description of the Prior Art
A disk device such as a floppy disk device has been popularly used in an office computer, a word processor and the like in addition to a personal computer and the application of the disk device has been spreading. This type of disk device is constituted as shown in FIG. 10, for example.
To explain the general construction of the disk device based on the drawing, in the drawing, numeral 101 indicates a chassis having a spindle center 102 as the center of rotation of a disk. The chassis 101 is housed in the inside of an equipment housing (not shown in the drawing) of a personal computer or the like, for example, and is formed in a bottomed box having a housing space which opens at a front side and an upper side as the entire configuration. A disk cartridge 103 faces the housing space.
A stepping motor 124 for feeding a head carriage and a head carriage which is arranged to be reciprocated in the back and forth direction by the stepping motor 124 are mounted on a rear end of the chassis 101. A first head 130 for reading recorded information on a disk is held at a distal end of the head carriage, while a head arm 132 having a second head 131 which corresponds to the first head 130 is tiltably mounted on a rear upper end of the head carriage by way of a resilient body. The head arm 132 is biased in the direction that the second head 131 approaches the first head 130. This type of disk device is further provided with a cartridge holder 136 for holding the disk cartridge 103 such that the disk cartridge 103 can be inserted and removed and a mechanism for opening and closing a shutter of the disk cartridge 103.
Here, as this type of disk device, to cope with the recent requirement for thin configuration of the disk drive, a disk device which is provided with an inner rotor motor shown in FIG. 11A and FIG. 11B as a motor for rotating a disk has been adopted.
This inner rotor motor is constituted of a stator 164 which includes an annular yoke 161 extending in the circumferential direction and a large number of cores 163 which are radially mounted on an inner peripheral surface of the yoke 161 and around which coils 162 are wound, and a rotor 166 which is rotatably arranged in an inner periphery of the stator 164 and having an annular magnet 165 which faces the cores 163 in an opposed manner. Further, in the drawing, numeral 168 indicates a circuit board on which a holding portion 170 which incorporates a bearing 169 therein is mounted, and numeral 171 indicates a rotary shaft for fixing rotor which is rotatably supported on the holding portion 170 on the circuit board 168 by way of the bearing 169 and has an axis extending in the vertical direction. Here, the rotor 166 of the inner rotor motor functions as a turn table which has a magnet for chucking disk (not shown in the drawing) and a rotary lever for chucking disk (not shown in the drawing).
With respect to the stator 164 for this type of inner rotor motor, the yoke 161 and the cores 163 are arranged to surround substantially the entire periphery of the rotor 166 except for a position where the heads 130, 131 move. To satisfy the required magnetic characteristics to these elements, they are formed of silicon steel which is costly compared to galvanized steel plate which constitutes the chassis 101 and the like.
However, with respect to this type of disk device, there has been a continuing demand for the reduction of the manufacturing cost. Further, the demand for making the device small-sized and light-weighted has been still strong.
Accordingly, the inventors of the present invention consider that there has been a demand for the reduction of areas of the yoke 161 and the cores 163 made of the expensive silicon steel with respect to the stator 164 for the inner rotor motor.
However, when the areas of the yoke 161 and the cores 163 are reduced to meet the abovementioned requirement, the magnetic interaction with respect to the rotor 166 becomes non-uniform in the circumferential direction so that there exists a possibility that the operational stability of the disk cannot be ensured.
Further, with respect to the inner rotor motor which is driven with three phases, for example, it is demanded to establish a state in which the coils which correspond to respective phases uniformly act on the rotor.
The present invention has been made in view of the abovementioned circumstances and has been provided to achieve following objects.
(a) Reduction of manufacturing cost
(b) Making device small-sized and light-weighted
(c) Maintenance of stability of rotation of motor
(d) Enhancement of operational stability of disk device
To achieve the abovementioned objects, the present invention provides an inner rotor motor comprising a rotor which includes a plurality of magnetic poles arranged circumferentially and a stator which is positioned outside a circumference of the rotor, has a stator core which includes a plurality of magnetic pole teeth which face the rotor in an opposed manner and arranges coils on respective magnetic pole teeth, wherein pitches of the magnetic pole teeth in the rotor circumferential direction along which respective rotor facing surfaces of the magnetic pole teeth are arranged are set smaller than pitches of the rotor in the rotor circumferential direction along which the magnetic poles of the rotor are arranged.
In the present invention, it is desirable that the pitches of the rotor in the rotor circumferential direction along which the magnetic poles of the rotor are arranged are preferably set 1.5 times greater than the pitches of the magnetic pole teeth in the rotor circumferential direction along which respective rotor facing surfaces are arranged.
The stator of the present invention is preferably arranged within 180xc2x0 with respect to a center angle of the rotor.
In the present invention, six magnetic pole teeth are preferably provided.
A disk device according to the present invention is provided with the abovementioned inner rotor motor.
According to the present invention, the pitches of the magnetic pole teeth in the rotor circumferential direction along which respective rotor facing surfaces of the magnetic pole teeth are arranged are set smaller than the pitches of the rotor in the rotor circumferential direction along which the magnetic poles of the rotor are arranged. Accordingly, compared to a case in which the magnetic pole teeth are provided with the pitches which are equal to or greater than the pitches of the magnetic poles of the rotor, the area of the magnetic pole teeth can be reduced and, at the same time, the area of portions which connect the magnetic pole teeth, that is, a yoke, can be reduced whereby the stator can be miniaturized and, at the same time, winding portions of the coils can be miniaturized thus leading to the reduction of the manufacturing cost. In other words, since the angular density of the arrangement of magnetic pole teeth with respect to the center angle of the center of rotation of the rotor can be set larger than the angular density of the arrangement of magnetic poles of the rotor, the size of the whole stator can be reduced while ensuring a fixed number of magnetic pole teeth of the stator.
Further, the coils of the present invention include U phase coils to which a first driving current is supplied, V phase coils to which a second driving current having a phase advanced by 120xc2x0 compared to the first driving current is supplied and W phase coils to which a third driving current which has a phase advanced by 120xc2x0 compared to the second driving current is supplied, and respective coils are arranged in the order of the U phase, the W phase and the V phase. Accordingly, the inner rotor motor of the present invention can control the driving currents in the same manner as a conventional three-phase motor and can drastically reduce the width of coils, that is, the area of magnetic pole teeth compared to the motor in which the U phase coils, the V phase coils and the W phase coils are arranged in the order of the U phase, the V phase and the W phase.
Further, according to the present invention, the pitches of the rotor in the rotor circumferential direction along which the magnetic poles of the rotor are arranged can be set 1.5 times greater than the pitches of the magnetic pole teeth in the rotor circumferential direction along which respective rotor facing surfaces are arranged. Accordingly, the width of coils can be set to two third of the width of coils of a conventional motor so that the inner rotor motor can be miniaturized and, at the same time, the area of the magnetic pole teeth can be reduced thus leading to the reduction of the manufacturing cost.
Further, in the present invention, the magnetic pole teeth can make the pitches of distal ends thereof in the circumferential direction which are expressed as center angles with respect to the center of rotation of the rotor to 15xc2x0.
Further, in the present invention, the stator is arranged in the range within 180xc2x0 with respect to the center angle of the rotor. Accordingly, compared to a structure in which the stator is provided along the entire periphery of the rotor as in the case of a conventional inner rotor motor, the area of the stator core can be reduced to approximately not more than half so that the cost incurred by the stator core made of silicon steel, for example, and the cost incurred by the winding of the coil or the like can be reduced whereby the manufacturing cost of the inner rotor motor can be reduced. At the same time, compared to the case in which the stator is provided to the entire periphery of the rotor, the area necessary for mounting the motor can be reduced so that the inner rotor motor can be miniaturized and can be light-weighted due to the reduction of the number of magnetic pole teeth.
Here, in addition to the state in which the stator is continuously arranged within the center angle of 180xc2x0, it is possible that the sum of center angles of portions where a plurality of stators which are arranged in a spaced apart manner is within 180xc2x0. Further, by arranging the stator in a range within 90xc2x0 with respect to the center angle of the rotor, further reduction of the manufacturing cost, further reduction of weight and further miniaturization of the inner rotor motor can be obtained.
In the present invention, by providing six magnetic pole teeth to the stator, the present invention is applicable to a three-phase inner rotor motor.